1. Field of the Invention
The present invention relates to a mobile satellite communication system and, more particularly, to a process of location registration for allowing a monitor section of the system to access location data of a mobile station which may be used for monitoring the mobile station.
2. Description of the Related Arts
Referring first to FIG. 1, there is shown a configuration of a conventional mobile satellite communication system using low orbit satellites. In the diagram, reference numeral 50 generally designates a conventional mobile satellite communication system using the low orbit satellites. The system 50 comprises a mobile station 1 able to move on the ground L and executing a location registration for informing a system monitor section of its location so as to be monitored based on the location registration data. The system 50 further comprises a plurality of satellite base stations 2a to 2c each moving along its orbit round the Earth and capable of communicating with the mobile station now being located within its service area (hereinafter, referred to also as control cell), and a plurality of earth switching centres 3 placed on the ground and constituting a communication network in cooperation with the satellite base stations 2a to 2c.
In the conventional mobile satellite communication system 50, the location registration is effected by means of base station ID's which are assigned to the satellite base stations 2a to 2c, that is, identification date on the satellite base station. More specifically, using the base station ID's, the mobile station 1 informs a system monitor section (not shown) of which one of a plurality of control cells 4 associated with the satellite base stations the mobile station 1 lies in.
Referring next to FIG. 2, there is shown a functional block diagram of a mobile station, a satellite base station, and an earth switching centre, which constitute the mobile satellite communication system. As a fundamental arrangement of a radio equipment, the mobile station 1 includes a transmitter/receiver section 12 having an antenna 11 connected thereto, and a communication control means 13 for controlling signals to be transmitted or received. Further, as the measures of effecting the location registration, the mobile station 1 includes a base station ID memory means 15, a comparison/update means 14, and a location registration means 16. The memory means 15 stores, as its location data, a base station ID of the satellite base station. The comparison/update means 14 compares the base station ID being stored with the base station ID derived from the satellite base station being in communication therewith, and if they are different from each other, updates the base station ID within the memory means 15 into a base station ID of the satellite base station. The location registration means 16 receives a comparison noncoincidence output from the comparison/update means 16, and transmits the mobile station ID to the transmitter/emitter section 12 by way of the communication control means 13.
The satellite base station 2a includes a transmitter/receiver section 22 having an antenna 21 connected thereto, a communication control means 23 for controlling the signals to be transmitted or received, and a base station ID output section 24 for outputting the base station ID signals serving as its identification data to the transmitter/receiver section 22. The earth switching centres 3 each include a transmitter/receiver section 32 having an antenna 31 connected thereto, a communication control means 33 for controlling the signals to be transmitted or received, and an arrangement not shown for transmitting or receiving signals with respect to the terrestrial telephone lines.
The action of the conventional system will be described hereinbelow. In this mobile satellite communication system 50, the mobile station 1, when energized, communicates with the satellite base station 2a managing the control cell 4 in which it lies. The mobile station 1 then compares the base station ID of the satellite base station 2a with the base station ID being stored within its base station ID memory means 15. If noncoincident, the location registration means 16 outputs the mobile station ID signal to the transmitter/receiver section 12 by way of the communication control means 13. The transmitter/receiver section 12, upon receiving this output, transmits the mobile station ID to the satellite base station 2a. When the satellite base station 2a is allowed to receive the location data of the mobile station 1, the new location data associated with the satellite base station 2a is registered within the monitor section of the system over the communication network of the system. Thus, the location registration procedure is carried out.
Thereafter, in the receiving mode, the movement of the mobile station 1 on the earth and the movement of the satellite base station 2a along the orbit round the Earth may cause a change of the satellite base stations to serve the mobile station 1. When the base station ID received by the mobile station 1 differs from the base station ID being stored within the memory means 15 of the mobile station 1, the same location registration procedure as the above is to be carried out.
In this manner, after energization of the mobile station 1, its location is constantly monitored by the monitor section of the system. For instance, when the mobile station 1 receives a call from a terrestrial telephone line (not shown), the monitor section of the system calls the mobile station 1 through the satellite base station 2a corresponding to the control cell in which the mobile station 1 lies at that time, over the communication network linking the earth switching centres 3 placed at various points on the ground with the satellite base stations 2a to 2c, to connect the terrestrial telephone line with the mobile station 1.
In this manner, the mobile satellite communication system 50 ensures that the mobile station 1 is connected to the terrestrial telephone lines irrespective of the terrestrial location of the mobile station 1.
It will be understood that although FIG. 1 depicts only three satellite base stations and a single earth switching centre for convenience of illustration, in the actual mobile satellite communication system, naturally, there will be provided more satellite base stations moving in orbit round the Earth sufficient to cover every area of the earth's is surface, and the earth switching centres are placed on the ground all important points in terrestrial telephone lines.
In the conventional mobile satellite communication system 50, however, since the satellite base stations 2a to 2c are moving along the orbits round the Earth, the location of the mobile station varies relatively when viewed from the satellite base stations 2a to 2c, even though the mobile station 1 is stationary. Thus, the location registration procedure is required whenever the satellite base station is substituted with another one, which will result in increased traffic for location registration.